Metallization of the3C−SiC(001)−(3×2)surface induced by hydrogen adsorption: A first-principles investigation

Abstract

We report ab initio calculations on the atomic and electronic structure, formation energies and surface vibrational modes of a variety of hydrogenated $3C\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}(001)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surfaces to shed more light on the recently discovered hydrogen-induced metallization of this semiconductor surface. The calculations are carried out employing the local density approximation of density functional theory. The adsorption systems are described using supercells, nonlocal norm-conserving pseudopotentials and Gaussian orbital basis sets. First, we show in agreement with other most recent ab initio calculations that Si dangling bonds at the third layer, originally suggested for explaining the experimentally observed surface metallization, are not stable. Instead, for low H exposure, we find H atoms to form monohydrides at the top layer and to become adsorbed in angular Si-H-Si bonds on the third layer. Due to these bonds the surface becomes metallic. We scrutinize further possibilities of surface metallization investigating numerous conceivable other H configurations on the first three layers of the $\mathrm{Si}\mathrm{C}(001)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surface. It turns out that H atoms can also occupy bridge positions in angular Si-H-Si bonds on the second layer yielding a locally stable metallic surface. For larger H exposure, we find H atoms to form dihydrides at the top layer and to become adsorbed on the second and/or third layer in angular Si-H-Si bridge bonds. These structures are metallic, as well, and they are energetically even more favorable. In all cases considered, the angular Si-H-Si bonds are the origin of the surface metallization giving rise to partially occupied surface state bands which close the gap and overlap in parts with the lower projected conduction bands. Finally we address surface phonons. Some of the calculated frequencies are in very good accord with experiment. Others, particularly characteristic for the Si-H-Si bridge bonds, have not been resolved in experiment, to date.